1. Field of the Invention
This invention generally relates to well cementing. More specifically, the invention relates to a methodology for selecting a cement composition to achieve long-term zonal isolation.
2. Background of the Invention
Zonal isolation refers to the isolation of a subterranean formation or zone, which serves as a source of a natural resource such as gas, oil, or water, from other subterranean formations. To achieve isolation of a subterranean formation, a well bore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass, thereby attaching the string of pipe to the walls of the wellbore and sealing the annulus. Subsequent secondary cementing operations such as squeeze cementing may also be performed.
Conventional cement slurry design for oil field applications involves optimizing slurry density, rheology, pump time, fluid loss, settling and gas migration mitigation during placement and compressive strengths for long term performance. Recently, properties such as tensile strengths, flexural strengths and elastic modulus have been employed as metrics to indicate long term performance. Other properties that are also important indicators for long-term performance of a cement sheath without failure include Poisson's ratio, frictional angle, and resistance to cyclic loading and unloading of pressure and temperature. During the life of the well, the cement sheath is subjected to detrimental cyclical stresses due to pressure and temperature changes. Measuring the cyclic stress value to which a cement sheath can be repeatedly subjected to may ensure that it can withstand a large number of cycles and provide zonal isolation during the life of the well. However, it is often found that no unequivocal correlation exists between different mechanical properties and cement sheath performance under cyclic stress conditions that would be expected in well operations over the life of the well.
Given the foregoing problems, it would be desirable to establish a correlation between different mechanical properties and long-term cement sheath performance. It would also be desirable to develop a methodology for designing a cement composition that can withstand the cyclical stresses that occur during the life of the well and is thereby suitable for long-term zonal isolation.